Mexican President Enrique Pena Nieto on Thursday canceled a planned Jan. 31 meeting with President Donald J. Trump, hours after Trump tweeted that the meeting should be scrapped if Mexico won't pay for a border wall.
Pena Nieto's message on Twitter ended days of uncertainty about how he would respond to Trump's aggressive stance toward the country, and illustrated the challenges world leaders are likely to face in dealing with Trump's voluble, Twitter-based diplomacy.
It also signaled a remarkable souring of relations between the U.S. and its immediate southern neighbor, one of America's . . .
